The Fish Fauna of Lake Victoria During a Century of Human Induced Perturbations

The Fish Fauna of Lake Victoria during a Century of Human Induced Perturbations Frans Witte1,2, Mary A. Kishe-Machumu1,3, Olivia C. Mkumbo4, Jan H. Wanink1,5, Pleun C. Goudswaard1,6, Jacco C. Van Rijssel1,2,7, Martien J.P. van Oijen2 ABSTRACT WITTE, F., KISHE-MACHUMU, M. A., MKUMBO, O. C., WANINK, J. H., GOUDSWAARD, P. C., VAN RIJSSEL, J.C. & VAN OIJEN, M. J.P. The fish fauna of Lake Victoria during a century of human induced perturbations. In J. Snoeks & A. Getahun (eds), Pro- ceedings of the Fourth International Conference on African Fish and Fisheries, Addis Ababa, Ethiopia, 22-26 September 2008. Tervuren: Royal Museum for Central Africa, ‘Zoological Documentation Online Series’, pp. 49-66. Lake Victoria, by area the largest tropical lake of the world, is well-known for its diverse native fish fauna, which com- prised about 500 endemic haplochromine cichlid species, two tilapiine species and 46 other species belonging to 12 families. During the past decades, the fish species diversity in the lake has declined dramatically due to human induced perturbations in the ecosystem. Based on literature and our own research findings we provide an overview of these changes and their most likely causes. During the first half of the last century, the increasing fishing pressure had a great impact on the native tilapiine cichlids and other large fish species. The shift in fish landings showed a classic example of fishing down the food web. Because of the dwindling catches, the Nile perch and four exotic tilapiine cichlids were introduced into the lake in the 1950s. Dramatic changes in the fish fauna occurred in the 1980s, with the upsurge of the introduced Nile perch and Nile tilapia, the decline of wetland zones, and increased eutrophication of the lake. The native tilapiines were replaced by the Nile tilapia, and several of the catfish species showed a dramatic decline, as did the lungfish. Most severely hit were the haplochromine cichlids, which disappeared from large parts of the lake, probably resulting in the extinction of many species. The introduced Nile perch and Nile tilapia, and the native cyprinid Rastrineobola argentea became the dominant fish species. Though water quality deteriorated and fish diversity decreased, fish landings rose from about 100,000 t y-1 in the 1970s to approximately 1 million t y-1 in the period 2005-2007. Since 1999 the biomass of Nile perch declined, whereas that of R. argentea increased. In the same period some of the sublittoral haplochromine species have recovered. Some of the surviving fish species, show remarkable changes in ecological and morphological features relative to the pre-Nile perch period, which seem to be adaptive responses to the changed environment. Although it may be possible to reconcile fisheries sustainability with biodiversity conservation in the lake basin, measures to reduce environmental stress in the lake are an urgent issue. Keywords: biodiversity, environmental degradation, eutrophication, extinction, fishery, haplochromine cichlids, Nile perch, speciation, species introductions 1 Institute of Biology Leiden, University of Leiden, P.O. Box 9505, 2300 RA Leiden, The Netherlands. 2 Naturalis Biodiversity Center, P.O. Box 9517, 2300 RA Leiden, The Netherlands. 3 Tanzania Fisheries Research Institute, P.O. Box 9750, Dar Es Salaam, Tanzania. 4 Lake Victoria Fisheries Organization, P.O. Box 1625, Jinja, Uganda. 5 Koeman en Bijkerk bv, Ecological Research and Consultancy, P.O. Box 111, 9750 AC Haren, The Netherlands. 6 IMARES Wageningen-UR, Institute for Marine Resources and Ecosystem Studies, P.O. Box 77, 4400 AB, Yerseke, The Netherlands. 7 Jacco C. Van Rijssel is the corresponding author: [email protected] 50 PROCEEDINGS OF THE 4TH INTERNATIONAL CONFERENCE ON AFRICAN FISH AND FISHERIES LAKE VICTORIA’S FISH FAUNA DURING THE PAST CENTURY – WITTE ET AL. 51 INTRODUCTION Table 1. Total number of fish species native to Lake Victoria arranged by family and number of species that With a surface area of 68,800 km2, Lake Victoria is were introduced into the lake. Sources: Greenwood 1966, 1974; Kaufman & Ochumba 1993; van Oijen 1995; the largest tropical lake in the world. It has a maxi- Seehausen 1996; Witte et al. 2007a. mum depth of 70 m, which is relatively shallow com- pared to other large lakes in East Africa (Fryer & Families/tribes Number Introduced Iles 1972). The lake is well known for its high fish of native species species diversity, dominated by haplochromine cich- species lids (Greenwood 1974; Seehausen 1996; Witte et al. Protopteridae 1 2007a). During the past century, dramatic anthropo- Mormyridae 7 genic changes were observed in the lake’s ecosystem. Alestidae 2 Fishery, species introductions and increasing eutro- Cyprinidae 17 phication had an enormous impact on the fish fauna of the lake (e.g. Ogutu-Ohwayo 1990a; Witte et al. Bagridae 1 1992; Verschuren et al. 2002; Hecky et al. 2010). The Schilbeidae 1 present paper aims to give a review of the changes in Clariidae 6 the fish fauna of Lake Victoria and to discuss the po- Mochokidae 2 tential causes of these changes. For this purpose we roughly divided the history of the lake over the past Nothobranchiidae 2 century into two periods: (1) before and during the Poeciliidae 5 1980s and (2) after the 1980s. Latidae 1 Cichlidae/ haplochromines ca 500 LAKE VICTORIA AND ITS FISH DIVERSITY Cichlidae/ tilapiines 2 4 BEFORE AND DURING THE 1980S Anabantidae 1 Fig. 1. Outline figures of representatives of the fish families in Lake Victoria. a, Protopteridae; b, Mastacembelidae; c, Poeciliidae; Mastacembelidae 1 d, Latidae; e, Cichlidae; f, Nothobranchidae; g, Mochokidae; h, Anabantidae i, Schilbeidae; j, Clariidae; k, Bagridae; l, Alestidae; m, More than 500 endemic haplochromine cichlid Mormyridae; n, Cyprinidae (after van Oijen 1995). species, all maternal mouth brooders, are known from Lake Victoria (Greenwood 1974; Kaufman & nid Rastrineobola argentea (Pellegrin, 1904) made Ochumba 1993; Seehausen 1996; Witte et al. 2007a). an important contribution to the fish fauna in the lake Furthermore, Lake Victoria used to harbour two na- (Okedi 1974). tive tilapiine cichlids, and 46 native non-cichlid spe- Lake Victoria haplochromines have been classified cies (Table 1), of which 16 are endemic to the lake into 15 (sub)trophic groups, each consisting of spe- and its drainage basin (Greenwood 1974; van Oijen cies sharing morphological characters related to the 1995). The native non-cichlid species belong to 12 capture, uptake, and processing of their dominant families (Table 1; Fig. 1). In the 1950s, the Nile food source (Greenwood 1974; Witte & van Oijen perch, Lates niloticus (Linnaeus, 1758), Nile tilapia, 1990). The distribution of trophic groups is habitat Oreochromis niloticus (Linnaeus, 1758) and three dependent. For instance, epilithic algae grazers are other tilapiine species were introduced into the lake restricted to rocky shores; insectivores and oral shell- (Welcomme 1988; Pringle 2005). ing molluscivores are mainly associated with hard Figure 3. Haplochromine trophic groups from Lake Victoria: (a) spe- From the end of the 1960s till the beginning of the substrates like sand and rocks; and detritivores are concentrated near mud bottoms (e.g. Greenwood cies composition in all habitats. Note that the paedophages and the scale 1980s, the haplochromine cichlids made up more scraper are included in the piscivores and that rare trophic types, like the than 80% of the demersal fish catches (Fig. 2b; Kud- 1974; Witte 1981; Witte et al. 1992; Seehausen et al. parasite feeders and a crab eater, are not included; (b) biomass composi- hongania & Cordone 1974). Other prominent species 1997b). tion in sub-littoral waters (6-20 m deep; from Witte et al. 2009a). in bottom trawl catches were Oreochromis esculen- With respect to the total number of species, the tus (Graham, 1929), O. variabilis (Boulenger, 1906), piscivores and insectivores were the most common Clarias gariepinus (Burchell,1822), Protopterus ae- groups (Fig. 3a), however, the detritivores and zoo- thiopicus Heckel, 1851 and Bagrus docmak Forsskål, planktivores were the most important with respect to Fig. 2. Changes during the 1970s and 1980s in (a) total fish landings in the Tanzanian part of Lake Victoria and b( ) demersal fish stocks in the Mwanza Gulf (Tanzania) calculated from bottom trawls catches. The decline of haplochromines in the bottom trawl 1775 (Kudhongania & Cordone 1974; Goudswaard biomass, at least in the sub-littoral habitat (6-20 m deep; Fig. 3b), and probably also in the open waters catches in the Mwanza Gulf in the period 1973-1984 was mainly due to trawl fishery; the subsequent further decline was caused & Witte 1997; Goudswaard et al. 2002a, b). Apart by the Nile perch upsurge. Note that bottom trawls did not catch the small pelagic Rastrineobola argentea. After 1985, Nile tilapia from these demersal fishes, the small pelagic cypri- of the lake. dominated the tilapiine catches (after Witte et al. 1999). 52 PROCEEDINGS OF THE 4TH INTERNATIONAL CONFERENCE ON AFRICAN FISH AND FISHERIES LAKE VICTORIA’S FISH FAUNA DURING THE PAST CENTURY – WITTE ET AL. 53 Several authors have raised doubts about the taxo- speciation through disruptive sexual selection for fishery in Lake Victoria. Perhaps with the exception 127 mm (5 inch) for gill nets was repealed in 1958, nomic status of Lake Victoria haplochromines (e.g. conspicuous coloration (Maan et al. 2004; Seehausen of scoop nets, netting material was not used in the which resulted

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